Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in length). In addition, the wind turbines are typically mounted on towers that are at least 60 meters in height. Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
Wind turbine blades have continually increased in size in order to increase energy capture. However, as blades have increased in size, it has become increasingly more difficult to control optimum energy capture. The blade loading is dependent on the wind speed, tip speed ratio (TSR) and/or pitch setting of the blade. TSR is the ratio of the rotational velocity of the blade tip to wind speed. It is important to optimize the operation of the wind turbine, including blade energy capture, to reduce the cost of the energy produced. Pitch setting of the blades (i.e., the angle of attack of the airfoil shaped blade), provides one of the parameters utilized in wind turbine control. Typically, controllers are configured to adjust rotor speed (i.e., the rotational speed of the hub around which the blades rotate) by adjusting the blade pitch in a manner that provides increased or decreased energy transfer from the wind, which accordingly is expected to adjust the rotor speed.
Wind turbines with sophisticated control systems maintain constant speed and power by active blade pitch control. Power production for a wind turbine is negatively impacted if the blades of the wind turbine operate in a non-optimal state. A common weather event that causes sub-optimal performance of the machine is blade icing in which appreciable amounts of ice collect on the leading edge or the upwind pressure surface. Ice build-up on the wind turbine blades reduce the efficiency of energy transfer from the wind and may ultimately result in aerodynamic stall from separation in airflow over the surface of the wing. In addition to icing, other forms of blade fouling may occur. For example, debris or insects may accumulate on the blades and reduce the aerodynamic efficiency of the blades. In addition, low air density or a drop in air density may also result in a loss of energy transfer from the wind to the blades.
Aerodynamic stall causes a decrease in lift and an increase in drag coefficients for a wind turbine blade. The onset of stall is signaled by a sharp change in a wind turbine's performance evident by degradation in output power versus expected power. In the event of aerodynamic stall, the energy transfer from the wind is reduced precipitously. Power degradation resulting from the loss of energy transfer is most significant during periods of rated winds where full power output is anticipated by the controller. That is, the control system interprets the decrease in power as a need for increased rotor torque. The control system reacts by calling for a decrease in blade pitch, which increases the angle of attack in an effort to increase the energy transfer from the wind. Increasing the angle of attack by the control system of an aerodynamically stalled blade further increases the flow separation, increasing the stall condition and further decreasing the energy transfer from the wind. As such, the current systems fail to address conditions, including icing conditions or similar conditions, such as low density air operation, blade fouling, or other blade condition susceptible to aerodynamic stalling.
Therefore, what is needed is a method for operating a wind turbine that optimizes energy capture by controlling the blade pitch angle for a large variety of operating conditions, including icing conditions.